A number of apparatuses have been disclosed for measuring displacement which utilize detection of shifts of light fringe patterns of diffracted light.
U.S. Pat. No. 5,355,220 Kobayashi et al discloses a light from a source radiated onto a diffraction grating to generate diffracted lights of different orders of diffraction permitting detection of light and dark stripes. Movement is measured by direct detection of movement of the stripes.
U.S. Pat. No. 5,486,923 to Mitchell et al discloses a grating which concentrates light having a preselected wavelength into + and - first orders while minimizing the zero order. The diffracted orders of light illuminate a polyphase detector plate.
U.S. Pat. No. 5,559,600 discloses a grating concentrating a preselected wavelength into positive and negative first orders. A polyphase periodic detector has it sensing plane spaced from the scale at a location where each detector element responds to the positive and negative first orders without requiring redirection of the diffracted light.
These devices do not provide the inherent precision as can be achieved with the present invention, and are generally expensive where fine resolution is required. The expense is due to the requirement to assemble these parts in precise locations relative to one another.